A magnetic clutch has a rotatable drive or input part with first magnets and a coaxially rotatable power output part with second magnets. The output part is magnetically coupled to the input part and the device has at least one sensor for measuring a magnetic field or its temporal differentiation, that is change in field strength with respect to time.
Devices and methods of the is type are known, for example from U.S. Pat. No. 6,467,944. With magnetic clutches, an angular offset appears between the input part with first magnets and the output part with second magnets that depends on the torque applied through the magnetic clutch. This angular offset is roughly proportional to the torque transmitted through the magnetic clutch. In US '944 two sensors are arranged to ascertain the magnetic fields of the two parts of the clutch. The first sensor detects the magnetic field of the input part and the second sensor that of the output part. According to the signals of the two sensors a time lag is ascertained, from which, in turn, the angular offset and thus the torque are determinable.
In such a system, therefore, two sensors have to be arranged to ascertain working parameters, from whose signal responses a time lag is derived to measure the transmitted torque. The provision of two sensors adds to manufacturing costs. Furthermore, it is expensive to calibrate such a two-sensor system, since two sensors and, thus, two measurement chains have to be calibrated.